1. Field of the Invention
The field of the invention is the computer modelling of steady-state groundwater flow in relation to collector wells, horizontal wells and galleries.
2. Description of the Related Art
A horizontal collector well is a well that is constructed by building a caisson that penetrates much of the vertical extent of an aquifer, then extending “lateral arms”, constructed of perforated pipe or wound well screens, into the aquifer. Compared to a traditional vertical well, a horizontal collector well provides much larger pumping capacity with less drawdown in head. Furthermore, the lateral arms of a horizontal collector well can be built very close to (or even underneath) surface waters, increasing the ability of the well to induce aquifer recharge from surface waters. As a result, collector wells and horizontal wells are commonly used in public drinking water supply or industrial water supply applications.
In water resource planning, it is important to understand the source of groundwater that is moving to wells, to be able to predict the quantity of water that can be removed from an aquifer system, to predict the effects of new groundwater development on existing facilities, and to understand the potential for contamination of well water. Computer models of groundwater flow are commonly used in these applications, but collector wells pose a particularly challenging problem. Since the amount of water that can be abstracted by a collector well is large, its effects on groundwater flow are expressed over a large regional extent; however, the performance of the collector well is greatly influenced by local factors, such as three-dimensional flow in porous media, resistance to flow into and out of surface waters, and head losses resulting from flow into the lateral arms and within the lateral arms.
We are aware of only a few published computational models of collector wells. In general, the prior art can be separated into three categories: (1) numerical models based on finite-difference or finite-element approximations; (2) fully three-dimensional models using analytical solutions; and (3) approximate two-dimensional solutions.
Finite-Difference and Finite-Element Approximations
A variety of numerical methods have been employed to simulate groundwater flow to collector wells. Many collector wells have been modelled by consultant with finite difference techniques (e.g., Sonoma County Water Agency, Louisville Water Company) but in each case the models are necessarily limited in the representation of regional flow, or in the representation of the near field conditions near the lateral arms of the collector.